Conventionally, a method of measuring a state of concrete which can optically detect chloride content contained in the concrete and a degree of neutralization thereof has been taught by, for example, JP 5031281 B (Patent Literature 1). The method of Patent Literature 1 detects deterioration of concrete based on absorption spectrum in a predetermined wavelength range of near infrared light reflected by a concrete building.
The method of Patent Literature 1 uses a wavelength range of 900 nm to 1700 nm of the absorption spectrum to detect concentration of calcium hydroxide so as to determine or measure the neutralization of the concrete, as the neutralization has correlation with the deterioration of the concrete. The method also uses a wavelength range of 1700 nm to 2500 nm of the absorption spectrum to detect concentration of chloride ion so as to determine or measure the salt damage, as the salt damage has correlation with the deterioration of the concrete.
According to the technique taught by Patent Literature 1, it is possible to accurately determine or measure the deterioration of concrete using a nondestructive method.